1. Field of the Invention
The present invention relates generally to image forming apparatuses using electrophotography, such as copiers, printers, facsimile machines, and multifunction machines having two or more of their functions, and developing units and process cartridges provided therein, and more particularly to a developing unit of the trickle development system, which suitably supplies new carrier into the developing unit, and a process cartridge and image forming apparatus including the same.
2. Description of the Related Art
There is a conventionally known technique that suitably supplies new carrier to a developing unit containing two-component developer formed of toner and carrier (in some cases, with an additive added thereto) in image forming apparatuses such as copiers and printers, which technique is referred to as the trickle development system. (See, for example, Japanese Laid-Open Patent Application No. 2001-183893.)
Toner is suitably supplied into the developing unit using a two-component developer through a toner supply opening provided in part of the developing unit in accordance with toner consumption in the developing unit. The supplied toner and the developer in the developing unit are agitated and mixed using a conveying member (an agitating member) such as a screw conveyor. Part of the agitated and mixed developer is supplied to a developing roller. The developer carried on the developing roller is restricted to an appropriate amount by a doctor blade. Thereafter, toner in the two-component developer adheres to a latent image on a photosensitive body drum at a position opposite the photosensitive body drum.
Thus, the carrier in the two-component developer contained in the developing unit remains in the developing unit without being consumed in a regular development process. Therefore, the carrier is degraded over time. In detail, the “film scraping phenomenon,” where the electrostatic charge capability of a carrier is reduced by the abrasion or separation of its coating layer due to lengthy agitation and mixing of the carrier in the developing unit, or the “spent phenomenon,” where the electrostatic charge capability of a carrier is reduced by adhesion of a toner component or additive to the surface of the carrier, occurs.
The trickle development system prevents degradation of the quality of an output image due to such carrier degradation over time. That is, this system maintains the amount and electrostatic charge capability of carrier contained in the developing unit by reducing a degraded portion of the carrier in the developing unit by suitably supplying new carrier (or new two-component developer) into the developing unit and suitably discharging part of the two-component developer contained in the developing unit from the developing unit.
Image forming apparatuses using this trickle development system achieve stabilization of the quality of an output image even over time compared with those requiring replacement of a developing unit or carrier with a new one every time there is degradation of the carrier with time.
Japanese Laid-Open Patent Application No. 2001-183893 describes a developing unit using the trickle development system, where overflow-type discharge means is employed for discharging a developer from the developing unit. In detail, a discharge opening (hole) is provided in the developing unit, and the developer (a portion made surplus by the supply of carrier) is discharged outside from the discharge opening when the surface of the developer conveyed to the position of the discharge opening exceeds a predetermined height.
According to the above-described developing unit of the trickle development system of Japanese Laid-Open Patent Application No. 2001-183893, when driving of the unit is started, the developer in the unit may be inclined to locally cause great undulations on its surface, so that there may be an unintended discharge of the developer. Repeated occurrence of such a phenomenon at every start and stop of the driving of the unit may cause an excessive discharge of the developer in the developing unit, thus causing a shortage of the amount of the developer.
This shortage of the amount of the developer in the developing unit causes the degraded condition of the developer to be unstable or the amount of electrostatic charge of toner to be reduced, thus causing a problem on an output image, such as a decrease in image density.